1. Field of the Invention
The present invention relates to apparatus and method for producing nonvolatile memories used as read-only storage media.
2. Description of the Related Art
Wireless door lock systems Where a door lock can be operated by remote control have been widely used in, for example, motor vehicles. A vehicle wireless door lock system is essentially composed of a key unit and a vehicle unit. The key unit comprises a code memory that stores code data (key codes) constituted by, for example, binary codes, and a transmitter for transmitting code data. The vehicle unit comprises a receiver for receiving the code data from the transmitter, and a drive device for operating a door lock in accordance with the code data received.
By operating the key unit, code data is read from the code memory and then transmitted by the transmitter. The code data is then received by the receiver of the vehicle unit which causes the drive unit of the vehicle unit to operate a door lock in accordance with the code data received.
It is desirable that the code data for one wireless door lock system be different from the code data for another system. In other words, the code memory of each wireless door lock system should store code data specifically assigned thereto.
The storage media normally used as the code memory that stores the forgoing binary data are PROMs (programmable read only memory) or mask ROMs (read only memory). One problem with mask ROMs is that they do not provide for a wide variety of binary code assignment. Namely, in the manufacturing of mask ROMs, a single reticle used in the mask formation step by a step repeater (or stepper) can contain only a few more than a dozen chip patterns. Hence, only a few more than a dozen binary codes can be assigned to the chips on a single wafer. Thus, different binary codes can be assigned to mask ROMs only on a group basis, not on an individual basis. In other words, use of mask ROMs does not allow for specific key code assignment to individual vehicles.
On the other hand, PROMs enable such individually specific key code assignment and therefore are often used as the storage media for the code memory. However, one problem with using PROMs is that the production process required for PROMs is about 1.5 times as long as that required for mask ROMs, resulting in increased production costs and slow delivery. Another problem with using PROMs is that the service life of PROMs is limited due to their relatively poor charge retaining characteristics. Hence, PROMs may suffer depletion or mutation of the written data over long period use.
Thus, there is a need for a storage medium that provides individual code assignment without substantially complicating the production process.